A current state of the art self restraining pipe joint is commonly known as the TR FLEX® Pipe joint as manufactured by U.S. Pipe and Foundry Co. Inc. This is described in U.S. Pat. No. 4,540,204 among others, which relies on individual locking segments engaging a weld bead on the spigot (male) end of one pipe with a cavity formed into the bell (female) end of the joining or mating pipe.
Several areas for improvement have been identified concerning the prior art involving this concept. The first is that the individual locking segments may not fully engage the weld bead placed on the spigot end of the pipe under all assembly conditions (including layout configurations and dimensional variations). These conditions may be so severe that the segments may, in fact, miss the weld bead entirely. This condition will greatly affect the thrust restraining capability of the joint. The second issue is that when the joint is deflected, the spigot approaches the bell at an angle instead of being concentric and parallel. This condition alters the longitudinal gap between the segment cavity (the bell groove) in the bell of one pipe and the weld bead on the spigot end of the other pipe. In short, the gap has a varying magnitude of separation. This gap is what is typically bridged by the individual locking segments. Only the locking segment(s) in the area of the smallest gap therefore will be engaged, creating a concentrated load on the bell and spigot of the mating pipe. The third issue is that assembly may be difficult in all but the most ideal circumstances since the individual locking segments must slide into the gap between the spigot and bell of mating pipes and this gap must be kept small in order to allow the segments to engage both halves of the pipe joint. The fourth issue is that the joint is sensitive to manufacturing tolerances. If the tolerances are allowed to vary too much, then the locking segments may not engage the spigot end of the mating pipe resulting in reduced joint performance.